Multiply.
Explanation: When a number is next to a variable, it means they are multiplied. So, $({6}{k^4})({-1}{k^2})$ is the same as $({6})({k^4})({-1})({k^2})$. Now we can rearrange the factors because multiplication is commutative (a fancy way of saying that the order in which we multiply things doesn't matter). ${(6)(-1)}{(k^4)(k^2)}$ Then simplify, and we're done! ${-6}{k^6}$